The invention relates to a communications device comprising a control unit and a delay unit, with which an acknowledge signal shifted in time relative to a received message can be transmitted.
In a mobile radio system like, for example, the C-network of the German Post-Office, the radio coverage area is divided into radio cells with respect to space and organisation. A stationary radio station generally located in the middle of such a radio cell provides the radio communication with all the mobile radio telephone sets located in this radio cell. The allocation of a mobile radio telephone set to a specific radio cell can be effected by means of distance measuring.
For this purpose, each stationary radio station receiving a request for establishing a call from a mobile radio telephone set sends a transmission request for an acknowledge signal. The period of time between receiving the request signal and sending the acknowledge signal is considered the response time. In order to obtain comparable results the radio telephone set has a fixed response time. The measured difference in time between receiving the request signal and sending the acknowledge signal provides the signal delay between the stationary radio station and the mobile radio telephone set after subtracting this response time. The distance between the stationary radio station and the mobile radio telephone set is computed from this signal delay. In order to avoid erroneous computations of the receiving instant, the response time is to be adhered to exactly.